1. Field of the Invention
The present invention relates to an optical disk comprising a printed layer on a signal readout surface side of the disk, for displaying material such as the content of the recorded information, and in particular relates to an optical disk facilitating the identification of visible information.
2. Description of the Related Art
In conventional optical disks, typically a printed layer is provided for displaying visual information such as figures and symbols so that the content of the recorded information can be readily understood.
For example, in the case of single sided recording reproduction type optical disks such as CDs (compact disks), a printed layer, which is formed by methods such as screen printing or offset printing, is provided onto the protective layer positioned on the disk surface reverse to the information signal readout surface, and hence almost the entire surface of the disk can be used for displaying figures and symbols.
In contrast, in the case of double sided recording reproduction type optical disks, onto which larger volumes of information are able to be recorded, the regions of the disk on which a printed layer can be provided are limited to those portions outside of the information recording regions such as the very narrow region close to the central portion of the disk, so as not to obstruct the readout of the information signal. Consequently the surface area which can be used for the display of figures and symbols has been very restricted.
In order to resolve the above problem, attempts have been made to enlarge the display area for visible information on double sided recording reproduction type optical disks, and examples include the inventions disclosed in Japanese Unexamined Patent Publication No. 9-81964, Japanese Unexamined Patent Publication No. 6-55887, and Japanese Unexamined Patent Publication No. 8-273201.
The inventions above comprise two optical disk base bodies each of which is formed by sequentially laminating an information signal layer of a translucent reflective film, a protective layer, and a printed layer onto an optical disk substrate, and which are then bonded together with the printed layers facing each other. Consequently, the information signal layer has a predetermined transparency, and the printed layer is visible from the disk substrate side of each disk base body which functions as the information signal readout surface, through the information signal layer and the protective layer.
With such a construction, a large surface area for the printing and display of figures and symbols can be ensured on both sides of the disk, without obstructing the readout of the information signal, even with double sided recording reproduction type optical disks.
Meantime, in those cases of viewing the visible information via the recording layer of the translucent reflective film by the naked eye, there can not be necessarily obtained excellent visibility, depending on the color arrangement of the printed layer. This is because, in viewing the printed layer on which is printed the visible information, through the recording layer of the translucent reflective film formed on pits based on signal information, it has become apparent that the visible light is attenuated on penetrating the translucent reflective film, hence, sufficient contrast of the dark/bright section is difficult, and due to projection resulting from reflection of the visible light from the translucent reflective film, and irregular reflections by the pits, the contrast of the dark/bright section decreases even further, and the translucent reflective film has its inherent color derived from the material and manufacturing method for this film.
The present invention takes the above factors into consideration, with an object of providing an optical disk capable of facilitating the visibility of visible information on a printed layer, by selecting such a color arrangement wherein the brightness difference between printed colors on the printed layer becomes 2.0 or above on the viewed surface of the printed layer.
Note, none of the aforementioned conventional technique have numerically referred to the color arrangement of printed colors on the printed layer.
In order to achieve the above object, an optical disk according to a first aspect of the present invention comprises a transparent disk substrate, a recording layer formed from a translucent reflective film which is layered on pits based on signal information provided on the disk substrate, and a printed layer which is layered on the recording layer and on which is printed visible information, and is constructed so that the visible information of the printed layer is visible from the disk substrate surface through the recording layer, wherein the printed colors of the printed layer are selected such that the brightness difference between the lightest section and the darkest section on the viewed surface of the printed layer becomes 2.0 or above.
According to such a constitution, when the printed layer is viewed from the disk substrate side, there can be sufficiently compensated for the attenuation of visible light on penetrating the recording layer of the translucent reflective film and the decrease of the contrast of the dark/bright section due to projection resulting from reflection of the visible light from the recording layer, and irregular reflections by the pits, to thereby allow the sufficient visibility of the visible information on the printed layer.
Furthermore, the optical disk of the present invention has a reflectance of 45%xcx9c55% relative to a reproduction light beam. By prescribing a reflectance relative to the reproduction light beam as 45% or above, as is prescribed in DVD standards, reproduction of optical disks using DVD players can be ensured. Moreover by setting the reflectance relative to the reproduction light beam to 55% or below, the visibility of the visible information on the printed layer can also be ensured.
In an optical disk disclosed in Japanese Unexamined Patent Publication No. 11-39723, because the sputtering time for the formation of an Au translucent reflective film was a relatively short 2.7 seconds, the reflective film was thin and the reflectance was low. Consequently, the reading precision of the information was lowered, making the disk unsuitable for use with DVD.
Furthermore, with the optical disk of the present invention, an optical buffer layer formed from a light transmissive material may also be provided between the recording layer and the printed layer.
According to the present invention, the material of the aforementioned recording layer may be selected from the group consisting of Au, an alloy incorporating Au, Cu, an alloy incorporating Cu, Si, an alloy incorporating Si as a main component, a compound of Si, and a mixture of Si and compound of Si.
According to the present invention, if the printed layer is constructed from a first printed layer for printing visible information, and a second printed layer for printing a background color for the visible information, then by selecting a suitable background color, the visibility of the visible information can be further improved. In such a case, if the second printed layer is formed by contact printing of a single color, then the printing process is relatively simple.
Furthermore, if the first printed layer and the second printed layer each have a thickness of 2 xcexcmxcx9c30 xcexcm, then not only is the visible information of the printed layer readily visible, but the effect by the printed layer on the electrical characteristics and the mechanical characteristics of the optical disk can also be kept to a minimum.
In order to apply the present invention to a double sided recording reproduction type optical disk, an optical disk in which the printed layer has the type of characteristics according to the first aspect of the invention may be used for at least one of a first optical disk and a second optical disk, with the first optical disk and the second optical disk then being bonded together with the respective substrates thereof facing outwards, thereby providing visible information on only one side of the double sided optical disk. Alternatively optical disks in which the printed layer has the type of characteristics according to the first aspect of the invention may be used for both a first optical disk and a second optical disk, with the two disks then being bonded together with the printed layers facing each other, thereby providing visible information on both sides of the double sided optical disk. In either case, if an opaque adhesive layer is provided between the first optical disk and the second optical disk to bond the two disks together, then the visibility of the visible information is able to be improved.
A double sided recording reproduction type optical disk of the present invention preferably has a reflectance of between 45%xcx9c55% relative to the reproduction light beam. Furthermore, an optical buffer layer formed from a light transmissive material may also be provided between the recording layer and the printed layer. Moreover, the printed layer may be constructed from a first printed layer for printing visible information, and a second printed layer for printing a background color for the visible information, with the second printed layer being formed by single color contact printing. In such cases preferably the first printed layer and the second printed layer each have a thickness of 2 xcexcmxcx9c30 xcexcm. Furthermore, the material of the recording layer may be selected from the group consisting of Au, Au alloy, Cu, Cu alloy, Si, an alloy incorporating Si as a main component, a compound of Si with either carbon or nitrogen, and a mixture of Si and compound of Si with either carbon or nitrogen.